Why utilities use Wye-Wye Conneted transformer?? 3

[JatTiw]

Hello,
I had two questions
1) Why utilities are using Y-Y connected transformer for power distribution? The only advantage I see is the cost (less insulation). Are there any other advantage/ disadvantage of using this type of transformer?
2) What are the disadvantages if both the star points (primary & secondary) are connected to the same grounding grid?

 

[allmend]

Our utility uses D/Yn (20 kV/0,4 kV) Transformers for power distribution. The 20-kV-Net (mostly cables) is compensated. The advantage is that you can locate and switch off the earth faults without interruption of the power supply.

 

[allmend]

„What are the disadvantages if both the star points (primary & secondary) are connected to the same grounding grid?“

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I do not see any disadvantages. If the high voltage and low voltage gear is installed in a common room, you even have to connect your star pints to avoid dangerous voltages between the two separate grounding grids (according to the German norm DIN VDE 0100-442).

 

[PWR]

The wye wye connection is least susceptible to ferroresonance during single pole switching which is a common practice for utilities.

Grounding both neutral points is the accepted practice and some transformers are built with the neutrals internally connected so that only one external grounding connection is required.

 

[jbartos]

Suggestion to JatTiw (Electrical) Mar 1, 2004 marked ///\\
1) Why utilities are using Y-Y connected transformer for power distribution?
///Y-Y transformers are used within power distribution networks on medium and high voltages since they are more advantageous than any other connections.\\ The only advantage I see is the cost (less insulation). Are there any other advantage/ disadvantage of using this type of transformer?
///Other advantages are considered to be:
1. The protective relay settings will be protecting better on the line to ground faults when the Y-Y transformer connections with solidly grounded neutrals are applied. The Y-Y network transformer standardization leads to numerous advantages.\\2) What are the disadvantages if both the star points (primary & secondary) are connected to the same grounding grid?
///All harmonics will propagate through the transformer, zero-sequence current path is continuous through the transformer, one line-to-ground fault will trip the transformer, etc.\\

 

[allmend]

„All harmonics will propagate through the transformer, zero-sequence current path is continuous through the transformer, one line-to-ground fault will trip the transformer, etc.“

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Please elaborate, Jbartos.

I cannot see any reason why zero-sequence current path should be continuous trough the transformer.

There is no big difference if the star points are connected beneath the earth surface or above the earth surface, provided you have an acceptable low impedance of the earth.

 

[dpc]

jbartos said:

"1. The protective relay settings will be protecting better on the line to ground faults when the Y-Y transformer connections with solidly grounded neutrals are applied. The Y-Y network transformer standardization leads to numerous advantages."

To my mind, protective relaying is MUCH easier on a delta-wye transformer because ground faults on the secondary side are isolated from the primary, making coordination much easier. If there is upstream relaying on a delta-wye transformer, any zero-sequence current can be assumed to be from a primary ground fault, allowing very sensitive ground fault protection. On a wye-wye, a low-side ground fault causes primary ground fault current, making coordination more difficult. Actually, ground fault protection is one of the primary advantages of delta-wye units.

Utilities like wye-wye transformers, as near as I can tell, because they are cheaper, it allows them to standardize and they provide protection against that old boogeyman ferro-resonance.

If I am designing an industrial installation, I will always try to negotiate for a delta-wye unit.

Old literature will also mention wye-wye units have problems with telephone interference due to third harmonics passing through the transformer, but I'm not sure if this is really much of an issue anymore.

 

[busbar]

It seems very much a regional thing, and is typical for multigrounded-neutral circuits. References are ANSI/IEEE C62.92.4-1991 …Application of Neutral Grounding in Electrical Utility Systems, Part IV—Distribution and IEEE C57.105-1978 …Application of Transformer Connections in Three-Phase Distribution Systems. In §7 of the second reference, Grd·Y—Grd·Y is listed as a configuration that will not be susceptible to ferroresonance. One place you may find it, where previously not used, is in a distribution-circuit uprating—say, from 12kV to 20.8kV. Overhead banks can be changed from delta to grounded-wye, as can padmount 3ø transformers equipped with a stick-operable primary oil switch.

Grd·Y allows for use of single-bushing-primary pole-top transformers.

 

[davidbeach]

One consideration for wye-wye as opposed to delta-wye, the transformer damage curve of a delta-wye is 58% of that of a wye-wye rating and impedance.

 

[dpc]

davidbeach,

You're right that consideration must be given to the delta-wye configuration when determining adequacy of transformer through-fault protection.

But the 58% figure you spoke of only applies for the condition of the the transformer primary protection being required to sense a line-to-ground fault on the secondary side. If you have secondary main protection, this 58% curve shift does not apply to the secondary protection.

The delta-wye transformer is just as rugged as a wye-wye, it's just that the primary protection does not see the full magnitude of fault current (on per unit basis) for a secondary ground fault.

 

[Skogsgurra]

jb has a point in pointing out that zero-sequence loads behave differently in wye/wye and delta/wye transformers.

This is more and more important as non-lnear loads are used. A wye/wye transformer has a high impedance to all zero-sequence currents (triplens) and will cause excessive voltage distortion with lots of third, ninth etc harmonics. A delta/wye transformer behaves a lot better in this respect. Delta/wye is used in most new installations in northern Europe.

 

[davidbeach]

dpc,

In close coupled applications I don't worry too much about the line-to-ground fault damage line as seen from the primary. But when the secondary main breaker is remote from the transformer, a cable fault in the secondary feeder has to be cleared by the primary device. Even in the close coupled case though, the wrench that was left resting against one bus bar, between the transformer and the secondary main, has to be cleared by the primary device. (Good work practices and mandatory tool inventory help here too.)

 

[allmend]

JatTiw (Electrical) wrote on Mar 1, 2004


2) What are the disadvantages if both the star points (primary & secondary) are connected to the same grounding grid?


jbartos (Electrical) answered on Mar 2, 2004


///All harmonics will propagate through the transformer, zero-sequence current path is continuous through the transformer, one line-to-ground fault will trip the transformer, etc.\\
skogsgurra (Electrical) wrote on Mar 2, 2004

jb has a point in pointing out that zero-sequence loads behave differently in wye/wye and delta/wye transformers.

-----------

skogsgurra,

JatTiw didn’t ask about the different grounding of wye/wye and delta/wye transformers, he just wanted to know, what is the difference between the disconnected and connected grounding grids in primary and secondary star points.

The zero sequence current will be continuous through the transformer even if the primary and secondary star points are connected to different grounding grids, IMHO.

 

[jbartos]

Suggestions to allmend (Electrical) Mar 2, 2004 marked ///\\\
„All harmonics will propagate through the transformer, zero-sequence current path is continuous through the transformer, one line-to-ground fault will trip the transformer, etc.“
---
Please elaborate, Jbartos.
///See for example Reference:
Stevenson "Elements of Power System Analysis" for explanations.\\
I cannot see any reason why zero-sequence current path should be continuous trough the transformer.
///I cannot see any reason why zero-sequence current path should be discontinuous in the grounded Y-Y transformer on the both sides.\\There is no big difference if the star points are connected beneath the earth surface or above the earth surface, provided you have an acceptable low impedance of the earth.
///About right.\\\

 

[brupp]

If you can obtain a copy, I recommend reading "The Whys of the Wyes", a General Electric publication numbered GET-3388B and dated 1967. That paper points out that the first 3-phase transformers were all wye-wye but that the numerous problems of this design have significantly reduced their use. Not the least of these problems are 3rd harmonic and high line-to-neutral voltage as zero load is approached. Also, in disagreement with JBartos above, wye-wye makes relaying quite a bit more difficult - not protection of the transformer but coordination of the relays in proximity to the wye-wye transformer.

Manufacturers often embed a delta tertiary winding in wye-wye transformers to overcome their inherent problems. Maybe this makes such a transformer more acceptable, but given a choice I would always choose a delta-wye unit. The only time I would consider a wye-wye transformer is when phasing dictated that choice.

 

[allmend]

To Jbarts:

„I cannot see any reason why zero-sequence current path should be discontinuous in the grounded Y-Y transformer on the both sides.“

----

Yes, in the grounded on both sides Y-Y transformer you have a zero sequence current in the primary and secondary winding.

But in this case there is no difference if the star points are connected to the same grounding grid or to separate grids.

That was my point.

 

[Skogsgurra]

allmend,

I hate to be like this, but I need to correct you about what JaTiw really asked about. He wrote:

JatTiw (Electrical) Mar 1, 2004

Hello,
I had two questions
1) Why utilities are using Y-Y connected transformer for power distribution? The only advantage I see is the cost (less insulation). Are there any other advantage/ disadvantage of using this type of transformer?

I answered question number 1. And what I say about distortion is right. There are also other reasons, like possibility to load the phases differently (talking about transformer without primary earth or neutral, which is how our tertiary lines are built over here).

 

[ALAK]

There is also an advantage of a zero phase shift between primary and secondary side. In a Yd or Dy connection there is always a plus or minus 30 degree phase shift(Dy11 or Dy1). A Yy vector group can be selected to match paralleling of transformers from different sources on the same bus.

 

[allmend]

„I answered question number 1. And what I say about distortion is right.“


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skogsgurra,

I am sorry if I misunderstood you. Though English isn’t my native language I had the impression that Jbarts commented on question Nr. 2.

JatTiw wanted to know: “ What are the disadvantages if both the star points (primary & secondary) are connected to the same grounding grid?“



As you see, JatTiw didn’t ask about the difference between grounded and ungrounded starpoints of Y-Y-Transformers, he just asked about the disadvantage of connection to the SAME grounding grid.

 

[allmend]

Speaking about grounding of star points.

Some German utilities are using YY transformers with a D-tertiary winding (for example 110 kV/20 kV), the primary star point is grounded, but not in every substation.

You have to reduce the fault current to 10 kA (to reduce the fault voltage on the grounding grid).

If you ground your star points you need a reliable interconnected grounding grid with a low impedance, otherways you get a dangerous fault voltage on your grounding grid.

This can be considered as a disadvantage.